javascript - 在 Flow 中传递通用参数对象

Question

我可以将类型作为通用参数传递，但是当我使用包含类型化参数的对象时，Flow 不理解也不建议类型。相反，我收到一个错误。

export type TAction<T, R> = { +type: string, payload?: T, meta?: R };

function thisWorks(action: TAction<string, boolean>) {
  const {
    payload,
    meta,
  } = action;
  // Flow knows payload is string and meta a boolean
}

function doesntWork(action: TAction<{ prop1: string, prop2: string }, boolean>) {
      const {
        payload: { prop1, prop2 },
        meta,
      } = action;
        // Flow doesn't know the type of prop1, prop2 or meta
    }

流错误是:

property prop1 is missing in TAction [1].Flow(InferError)

Answer 1

我已经创建了流程来尝试说明上述问题:Try it here

从中我可以看出，问题是在 TAction 中，meta 和 payload 都被注释为 optional properties .这就是为什么在解构 payload 之前需要对其进行检查，确保它不是 undefined。像下面这样的代码不会有流程错误:

export type TAction<T, R> = { +type: string, payload?: T, meta?: R };

function thisWorks(action: TAction<string, boolean>) {
  const {
    payload,
    meta,
  } = action;
  // Flow knows payload is string and meta a boolean
}

function doesntWork(action: TAction<{| prop1: string, prop2: string |}, boolean>
    ) {
      const {
        payload,
        meta,
      } = action;

      if (payload) {
        const  { prop1, prop2 } = payload;
        // Do whatever is needed
      }
    }

Try with flow

或者可以更改 TAction 的注释，因此 payload 和 meta 将不再是可选属性，如下所示:

export type TAction<T, R> = { +type: string, payload: T, meta: R };

function thisWorks(action: TAction<string, boolean>) {
  const {
    payload,
    meta,
  } = action;
  // Flow knows payload is string and meta a boolean
}

function doesntWork(action: TAction<{| prop1: string, prop2: string |}, boolean>
    ) {
      const {
        payload: { prop1, prop2 },
        meta,
      } = action;

        // Flow doesn't know the type of prop1, prop2 or meta
    }

Try with flow